DTV

I had some questions about DTV and was given a tour of WJEB studio and transmitter site. Clay Roney, KI4FWC is the chief engineer showed me around and answered many questions I had about DTV.

I asked what happened with all of the obsolete analog transmitters.Answer: Most went into the dumpsters as there was no market for them as even third world countries have converted to DTV and since transmitters are built for reliability and most had many hours on them anyway.

During the transition time when analog and DTV was being transmitted at the same time, how could there could be two different transmitters on the same frequency without interference? Answer: They didn't transmit on the same frequency. With digital transmissions they were transmitted on a different channel from the analog channel and embedded in the digital signal is a tag that tells the receiver that this is the new frequency for the new DTV broadcast. For example channel 59 transmits on 44, 30 on 32, 7 on 38, 17 on 34, 4 on 42, 47 on 19, 12 on 13, and 25 on 10.

How many streams (sub channels) can a station transmit? Answer: There is a practical limit of 5. Every stream takes away from the possible resolution of all of its sub channels streams. Each channel has 6 Mhz bandwidth and the main stream gets 1.72 Mhz with the main stream getting the most bandwidth for the best picture quality. The rest is divided between the other streams.

Does having more than one sub channel take away from the power of the main channel like subcarrier authorization (SCA) takes away from FM radio broadcasts? Answer: No, DTV uses multiplexing.

For viewers that are receiving DTV broadcasts off the air in rural areas, has your coverage area increased? Answer: Our coverage area stayed the same for the most part. The "cliff effect" (you either receive us or you don't) has not been a problem. Switching antennas gave us a more kidney shaped coverage area. I noticed that some antennas are tilted downward 1-3 degrees so that nulls near the transmit site are eliminated.

I have noticed that some stations transmit at 480 I and some transmit at 1080 I. Can any digital transmitter handle 1080 I? Answer: Yes, 1080 I is high definition. An analog video must go through an analog to digital converter in order to transmit a digital picture.

I have noticed that several DTV transmitters will use the same tower with each antenna being only a few feet separated from another with thousands or even a million of watts effective radiated power coming out of each antenna. Are there any problems of arcing, cross talk, or intermod? Answer: No, the transmit frequencies are different enough and we use cavities.

One of the towers in Jacksonville is about 55 years old and the bucket elevator has had its share of stories about its operation. What is the useful life of a commercial tower? Answer: If maintenance is done on a regular basis, guy cables are greased, bulbs and strobes are changed regularly, these towers will last a long time. Hardline weight and wind loading is a concern on older towers.

Are the power stages still using tubes? Answer: Tubes are still used today. Klystrons which were popular in the past will not handle the bandwidth necessary for DTV. Today IOT's are used.

These are water cooled with a water- antifreeze solution to prevent corrosion. These tubes are expensive and cost $50,000 to replace and these cannot be rebuilt. We do not keep a spare because of the cost but we run them at reduced power so they will last a long time in thousands of hours. Transmitters have a very long life and can easily last twenty years or more.

Some of the manufacturers have downsized or sold out because there will not be much need for new equipment for a long time as the rush to DTV is over. Harris and Thales have many transmitters in operation today. There appear to be a large amount of JEA power lines in the Hogan Road area. Our transmitter uses 480 volts and three-phase current.

Do engineers from competing stations borrow parts from competing stations? Answer: We borrow from others and we help others. Our job is to keep the station on the air and not lose viewers.

What education does a station manager-owner look for in a broadcast engineer? Answer: Although the FCC does not look at the engineer for proper operation of the transmitter, they hold the licensee responsible. Many universities offer a degree in broadcast engineering.

Do you climb the tower for maintenance? Answer: No. Specialists called Tower Dogs do this.

Have there been any higher incidence of health problems cataracts, brain cancer, etc. for engineers working around high levels of RF? Answer: No, as the RF is concentrated on the antenna.

Does the FCC come around? Answer: Stations hire a private inspector to go around making a surprise visits to make sure everything is right. Next he will send a letter to the FCC just before renewal time that everything meets FCC rules. Interference to locals would bring the FCC in for an inspection. At this station all cable runs were tagged and there are binders with all schematics and cable runs.What happened to Mobile TV? Answer: It fell by the wayside.

Thank you to Clay Roney, KI4FWC and WJEB for and informative tour and answering all my questions﻿.

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Traveler Information Radio

By Ross Goodall WD4NJV

While traveling one will come across various signs advising motorists to tune into 530 KHz or 1610-1700 KHz for travel information. Sometimes traveler Information Service (TIS) radio is called Highway Advisory Radio (HAR). Some signs specify for motorists to tune in when lights attached to the sign are flashing. Some of these transmit continuously such at Jacksonville International Airport and some transmit only when travel information warrants. Under ideal conditions the Jacksonville International Airport’s broadcasts on 530 Khz can be heard as far away as the Regency Square area.

When in North Carolina I stopped at the welcome station on I-26, south of Asheville and saw an interesting transmitter site that is used to transmit traveler information. It was located at the edge of the parking lot and had an approximately 25 foot whip antenna alongside a stainless steel box and a 20 inch whip antenna mounted on top of the box. There were four ground radials coming out of the box. The 25 ft. antenna had a short section of PVC pipe over the lower part of the antenna so someone touching it would not receive an RF burn. I think the short antenna was used to receive signal from a control site to turn on this remote transmitter as needed.

Researching this on the Internet, I found that the station is WNXE972 licensed to the North Carolina Public Safety Pool. This station only transmits when conditions warrant such as dense fog which is a problem in the mountains, hazardous road conditions, and traffic accidents.

For a list of TIS stations FCC.gov and then search Travelers’ Information stations.

The FCC grants licenses to government entities, park districts, and authorities for a purpose of disseminating information by AM broadcast radio to travelers. The FCC authorized service in 1977 on assigned frequencies of 530 KHz or 1610-1700 KHz and is limited to 10 watts our output power and an antenna height limited to 15 meters (49.2 ft.) No commercial information may be transmitted and no licenses are granted to individuals or groups.TIS is used in other countries.